A father who fatally shot his two teenage children at a home in Sydney’s northwest before shooting himself at a nearby house had been involved in a long-running custody battle, police say.

The 68-year-old gunman’s body was found in a Normanhurst home, five kilometres from the scene of the initial killings, about 12 hours after his two children were found murdered, NSW Police confirmed on Friday.

The man was the father of the children but did not live at the West Pennant Hills home, police said.

“The 68-year-old male was the holder of a number of firearms. The two handguns were registered in his name,” NSW Police Acting Region Commander Brett McFadden told reporters in Sydney on Friday.

“We understand that those handguns came into his possession lawfully earlier this year,” he said.

Police say they were “aware” of the father.

The teenagers had been at home by themselves, police say.

The 15-year-old boy and a 13-year-old girl were found dead in their bedrooms at a house in Hull Road at West Pennant Hills shortly before 5.30pm on Thursday.

Their mother, 36, arrived shortly after police and was treated by paramedics for severe shock.

“The mother resided at the address at Pennant Hills with the two children. The 68-year-old did not reside at the house. He had to attend to commit the crime,” Det Supt McFadden said.

“I do know there were some custody hearings over the last two years.”

Massive manhunt for father

The discovery of the man’s body followed a massive manhunt overnight, launched after police found the murdered teenagers.

“When ambulance paramedics arrived … we were confronted with an extremely distressing and stressful situation,” NSW Ambulance Inspector Kevin Sweeney told reporters at the scene of the shooting.

“Two teenagers were unfortunately deceased in their bedroom and unfortunately at that time there was nothing we could do to help them.”

It’s unknown how many shots were fired with detectives speaking to several witnesses in an attempt to understand what happened.

Mr McFadden said emergency services had arrived to an “horrific scene”.

“It was a very harrowing experience for the emergency services who responded,” he said.

“You’ve got a family that is going to struggle to come to terms with it and emergency responders will struggle to come to terms with what they have seen.”

An extensive forensic examination led by the homicide squad is under way.

Police benefit from witness accounts

A resident who lives just 50 metres from the house, John Howard, says he’s shaken.

“I could hear the police helicopter and then I saw it all over the television,” he told AAP.

“It’s a friendly, quiet street.”

Another resident, who described the area as friendly and safe, said it was an “unbelievable tragedy”.

“I feel so shocked. Kids are innocent. Whatever the reason, kids shouldn’t suffer,” she said.

If you or anyone you know is experiencing difficulties, help is available. Contact Lifeline 13 11 14 or beyondblue 1300 22 4636.

-with AAP